1. Field of the Invention
The present invention relates to a voice coil motor, and more particularly, to a voice coil motor with a storage medium storing a gain value and an offset compensation value.
2. Description of the Prior Art
In order to perform a more accurate position control for a set of lenses or an access arm of a hard disk drive, a voice coil motor (VCM) is usually adopted to move the set of lenses or the access arm of the hard disk drive. The structure of a VCM is primarily a coil placed within a magnetic circuit including a permanent magnet.
In an optical system employing a VCM, there is an interaction propelling force between the coil and the permanent magnet according to Fleming's left-hand rule that moves a carrier physically connected to the permanent magnet when a current flows through the coil; at the same time, a set of lenses which is attached to the carrier is moved correspondingly, so as to achieve optical zoom and focus functionalities. Based on applying a specific current value through the coil, an accurate control for optical zooming and focusing can be achieved.
In general, in order to accurately control the lens carrier of a voice coil motor, one must have a clear knowledge of the location of the lens carrier, and Hall sensors are used to perform such detection.
FIG. 1 schematically illustrates a diagram of a VCM system of the prior art. As shown in FIG. 1, the VCM system comprises a computing platform 10, a VCM 11, a controller 12, and a Hall sensor 13. The controller 12 and the Hall sensor 13 are integrated together into the VCM 11. The Hall sensor 13 outputs a voltage signal via an output terminal as long as the Hall sensor 13 detects magnetic field strength when a lens carrier (not shown) physically connected with a magnetic device (not shown) is moved accordingly, and then the controller 12 determines the current location of the lens carrier based on the magnetic field strength.
In mass production, however, it is almost impossible to provide a magnetic device with the exact characteristics that would maintain absolute consistency of relative position after the majority of components in the VCM 11 are assembled resulting in the VCM 11 having slightly different control features. That is, when there are different VCMs 11 with lens carriers in the same position, the Hall sensor 13 detection of the magnetic field strength may not be the same.
When the magnetic strength of the magnetic device inside the VCM 11 is smaller or the gap between the magnetic device and the Hall sensor 13 is relatively large, this only enables the Hall sensor 13 to detect a magnetic field strength smaller than the theoretical value. When the magnetic strength of the magnetic device inside the VCM 11 is larger or the gap between the magnetic device and the Hall sensor 13 is relatively small, this only enables the Hall sensor 13 to detect a magnetic field strength larger than the theoretical value.
To solve the problem, a compensation scheme, such as an adjustable gain value and an adjustable offset compensation value relating to the amplifier, can also be used in the controller 12 to allow each VCM to have the same operation mode. The gain of the amplifier is raised to compensate for variation if the magnetic field strength detected by the Hall sensor 13 is lower than the theoretical one. The gain of the amplifier is reduced to compensate for the variation if the magnetic field strength detected by the Hall sensor 13 is higher than the theoretical one.
However, the compensation scheme employed by the controller 12 wherein an amplifier compensates the variation using the adjustable gain value and the adjustable offset compensation value, has some problems in the prior art. Firstly, a manufacturing volume of VCM extracts an optimal amplifier gain value and an offset compensation value, and these values are used as a standard to test all the VCMs during mass production. This method will increase the amount of defective VCMs and lower the yield rate because the tolerance variation among VCMs is usually large in mass production. Secondly, the gain value and the offset compensation value of the amplifier of the VCM are extracted from each VCM in the manufacturing process one by one. Although this method will increase the amount of good VCMs and yield rate, the user, such as a camera module manufacturer or a phone manufacturer, needs to detect the gain value and the offset compensation value appropriate for the amplifiers one by one.